Support multiple -m flags for multi-line bodies

[remenoscodes]

git commit supports multiple -m flags to build multi-paragraph messages:

git commit -m 'First paragraph' -m 'Second paragraph'

git-issue-create and git-issue-comment should support the same pattern:

git issue create 'Bug title' -m 'Context paragraph' -m 'Steps to reproduce'

Currently, each -m overwrites the previous value. Instead, multiple -m flags should be concatenated with a blank line separator (matching git commit behavior).